EP-A-1 273 640 describes coating compositions comprising a polyol component and a crosslinker component consisting of aliphatic and/or cycloaliphatic polyisocyanates, 0.1 to 95 mol % of the free isocyanate groups originally present having undergone reaction with bisalkoxy-silylamine. As catalyst these coating compositions comprise customary catalysts, such as organotin compounds, dibutyltin dilaurate for example, amines or sulfonic acid-based catalysts, p-toluenesulfonic acid, for example. The coating compositions described in EP-A-1 273 640 can be used for OEM finishing and exhibit good scratch resistance in conjunction with good stability toward environmental influences. However, these coating compositions have a particularly strong tendency toward post-crosslinking, since conversion on thermal curing after application is inadequate. This has adverse effects especially for the weathering stability.
WO2008/074489 discloses coating compositions which comprise at least one hydroxyl-containing compound (A) and at least one isocyanato-containing compound (B), with one or more constituents of the coating composition containing hydrolyzable silane groups. It is essential to the invention that the coating compositions described therein comprise as catalyst (D), for the crosslinking of the silane groups, phosphorus-containing catalysts, more particularly amine-blocked partial ethylhexyl esters of phosphoric acid. Examples of amines with which the phosphoric esters are blocked are tertiary amines, preferably triethylamine. The coating compositions described therein lead to coatings having a high scratch resistance in conjunction with good weathering stability. The coating compositions are to be improved still further, however, in respect of their optical properties, more particularly in respect of their appearance. Moreover, the electrical resistance of these coating compositions is inadequate.
Especially if the coating materials are electrostatically charged in the course of their application, the electrical resistance of the coating materials is of particular importance. Depending on the plant, it may be necessary on safety grounds for the coating materials, and also, in the case of 2-component coating compositions, the individual coating components, not to have less than a particular electrical resistance value. Against this background there is interest in providing coating materials which do not fall below a certain minimum level in respect of electrical resistance. In the field of automotive OEM finishing, therefore, the usual requirement is for the coating compositions or, in the case of 2-component coating compositions, the individual coating components to have an electrical resistance of at least 200 kohm, preferably at least 250 kohm, and more preferably at least 350 kohm. The limiting values acquired are each plant-specific limiting values and may vary according to the specific mandates of the plant manufacturers. The highest limiting values in each case are particularly preferred on account of the fact that they meet the mandates of a variety of plants.
The electrical resistance is investigated with the aid of a dip probe measuring cell, with a conductivity meter of type LC 2 from Byk Gardner, in accordance with DIN 55667 at 25° C.
Finally, the as yet unpublished German patent application DE 102008 060454.2-43 describes coating compositions comprising at least one hyperbranched, dendritic, hydroxyl-functional polyester (A) having an OH number ≧180 mg KOH/g as measured via DIN 53240, and also polyisocyanates (B) which comprise at least partly hydrolyzable silane groups. Used as catalyst (D) for the crosslinking of the silane groups are phosphorus-containing catalysts, more particularly amine-blocked partial ethylhexyl esters of phosphoric acid. Examples given of amines with which the phosphoric esters are blocked are tertiary amines, such as dimethyldodecylamine or triethylamine, and also bicyclic amines, such as diazabicyclononene (DBN), diazabicyclooctane (DABCO), and diazabicycloundecene (DBU), for example. Information as to how the electrical resistance of the binder mixtures and of the coating compositions can be set, however, is absent from that specification. Nor is there any description of the use of amines comprising at least one branched aliphatic hydrocarbon radical for the purpose of blocking the phosphoric esters.
It was an object of the present invention, therefore, to provide binder mixtures and coating compositions produced from them, particularly for the clearcoat in OEM finishes and automotive refinishes, that lead to coatings having a combination of high scratch resistance, good acid resistance, and good weathering stability, with at the same time a very good overall appearance.
The aim was therefore to provide coating compositions which lead to a network with a high degree of weathering stability and which at the same time ensure high acid resistance. In addition, the coating compositions are to lead to coatings which already have a high degree of scratchproofing straight after thermal curing, and in particular a high level of gloss retention after scratch exposure. Moreover, the coatings and coating systems, especially the clearcoat systems, ought to be able to be produced even in film thicknesses >40 μm without stress cracks occurring. This is a key requirement for the use of the coatings and coating systems, particularly of the clearcoat systems, in the technologically and esthetically particularly demanding field of automotive OEM finishing.
At the same time the binder mixtures and the coating compositions produced from them ought not to fall below a certain minimum level in respect of their electrical resistance. In the field of automotive OEM finishing a customary requirement is that the electrical resistance not only of the individual coating components, such as the binder mixture, but also of the coating compositions produced from them be at least 200 kohm, preferably at least 250 kohm, and more preferably at least 350 kohm.
Finally the coating compositions ought, however, also to exhibit good transparency (low haze values) and good leveling and lead to coatings having a good overall appearance.
In the light of the above objectives, binder mixtures based on aprotic solvents have been found, comprising
at least one hydroxyl-containing compound (A) and
at least 1.0% by weight, based on the nonvolatile constituents of the mixture, of at least one phosphorus- and nitrogen-containing catalyst (D) for the crosslinking of silane groups,
wherein the mixture comprises
    (i) as hydroxyl-containing compound (A) at least one hydroxyl-functional polyester (A) in which on average at least one hydroxyl function of the polyester is esterified with at least one acid selected from the group of the isomeric C8 to C9 monocarboxylic acids, and    (ii) as catalyst (D) at least one phosphorus-containing catalyst blocked with at least one amine of the formula (I)
                where        R1 is an acyclic aliphatic or araliphatic hydrocarbon radical having at least 3 C atoms,        R2 is an acyclic aliphatic or araliphatic hydrocarbon radical which is the same or different from R1 and/or R3, and        R3 is hydrogen or an acyclic aliphatic or araliphatic hydrocarbon radical which is the same or different from R1 and/or R2.        
The invention further provides coating compositions based on the binder mixture, and also multistage coating methods using these coating compositions, and also the use of the coating compositions as clearcoat material and application of the coating method for automotive OEM finishing and automotive refinish, and also for the coating of parts for installation in or on automobiles.
In the light of the prior art it was surprising and unforeseeable for the skilled worker that the objects on which the present invention was based could be achieved by means of the binder mixture of the invention and by means of the coating composition of the invention based on said binder mixture.
Thus it is particularly surprising that the binder mixtures of the invention and the corresponding coating compositions of the invention, in spite of the relatively high fraction of catalyst (D), have an electrical resistance of at least 200 kohm, preferably at least 250 kohm, and more preferably at least 350 kohm, as required in the automotive OEM finishing segment.
At the same time the coating compositions of the invention produce new coatings and coating systems, especially clearcoat systems, which are highly scratchproof and, in contrast to common, highly crosslinked scratchproof systems, are acid-resistant. Moreover, the coatings and coating systems of the invention, especially the clearcoat systems, can be produced even in film thicknesses >40 μm without stress cracks occurring. Consequently the coatings and coating systems of the invention, especially the clearcoat systems, can be used in the technologically and esthetically particularly demanding field of automotive OEM finishing. In that context they are distinguished by particularly high carwash resistance and scratch resistance. The high scratch resistance of the coatings exists in particular directly after their final curing, and so the coatings can be handled without problems immediately after final curing.
Furthermore it is surprising that the binder mixtures and the coating compositions at the same time also exhibit good transparency (low haze values) and good leveling and lead to coatings having a good overall appearance. Finally, the components according to the invention can be prepared with particular ease and with very good reproducibility and on application of the coating material cause no significant toxicological or environmental problems.